GolfScript - 243 232 228 characters

I translated my CoffeeScript answer into GolfScript, which is much more suited to the string manipulations.

EDIT: Saved six characters by properly using the increment operator, three by making good use of the stack, and six more by irresponsibly redefining operators I'm not using.

Entirely golfed:

..0="HL"?2+3%:o)2%.@="CDEFGAB"?7o*+:`2%45 32if:r;
).2$,<{=}{;;r}if:&;
[" "5*:|" --- ":g]4*[|"-"5*]5*+[|g|]+.
[`<~]\[`>([0=:^&79r^]''+\~]
+17`<`)18if<9`>`9if:j>:t 13`>.2*):x;
4,1>{`j-\2${+}{-}if}%\;
{.@<\t>(:v[x<'|'+x)v>+]\++:t}
/-1%n*

With comments:

# extract octave
..0="HL"?2+3%:o

# extract note
2%1\-.@="CDEFGAB"?7o*+:k

# line spacer
2%45 32if:r;

# extract accidental
1+.2$,<{=}{;;r}if:a;

# staff
[" "5*:|" --- ":g]4*[|"-"5*]5*+[|g|]+.

# lines below
[k<~]\

# note line and above
[k>([0=:w a 79r w]''+\~]+

# cut off just what we need
17k<1k+18if<
9k>k 9if:j>:t;

# and the note stem
13k>.2*1+:x;4,1>{k j-\2${+}{-}if}%\;

{
  .t<\
  t>(:v[x<'|'+1x+v>+]\++:t;
}/

# now output the note
t-1%n*